Solid state lighting (SSL) is rapidly becoming the norm in many lighting applications. This is because SSL elements such as light emitting diodes (LEDs) can exhibit superior lifetime and energy consumption, as well as enabling controllable light output color, intensity, beam spread and/or lighting direction.
Tubular lighting devices are widely used in commercial lighting applications, such as for office lighting, for retail environments, in corridors, in hotels, etc. A conventional tubular light fitting has a socket connector at each end for making mechanical and electrical connection to connection pins at each end of a tubular light. Conventional tubular lights are in the form of fluorescent light tubes. There is a huge installed base of luminaries equipped with electronic ballasts for fluorescent tube lamps.
There are now tubular LED (“TLED”) lamps which can be used as a direct replacement for traditional fluorescent light tubes. In this way, the advantages of solid state lighting can be obtained without the expense of changing existing light fittings. Indeed, TLEDs that are compatible with fluorescent lamp ballasts are the most straightforward and lowest cost way of replacing fluorescent lighting by LED lighting. Both rewiring (removing the ballast, connecting a TLED directly to AC mains) and replacing the whole luminaire are considerably more cumbersome and expensive. It may be advantageous to provide a TLED that is compatible with the existing luminaire and the ballast therein. Both electromagnetic (EM) and electronic high frequency (HF) ballasts are used in fluorescent lighting.
FIG. 1 shows a typical block diagram of a TLED that is compatible with a fluorescent ballast.
The ballast 10 comprises a half-bridge parallel resonant converter and it drives an electronic (high frequency) ballast compatible TLED 12.
The ballast 10 and high frequency compatible TLED 12 are connected via the connection pins 1 and 2 at one end of the TLED and via the connection pins 3 and 4 at the other end of the TLED.
A high frequency compatible TLED 12 typically comprises all of the building blocks depicted in FIG. 1. These are a filament emulation unit 14, a pin safety and start-up circuit 16, a matching circuit 18, a rectifier 20, an LED driver 22, a smoothing capacitor 23 and the LED string 24. The LED string 24 extends between an internal ground 25 and a high voltage DC bus 26.
For most of these building blocks, the implementations shown in FIG. 1 are just examples and other implementations of their functions are possible and are also used.
The details of the design of the half-bridge ballast 10 are not shown in FIG. 1. This type of ballast is also just an example and other implementations such as push-pull converters are also possible and in use.
The LED driver shown in FIG. 1 is a shunt switch driver. In this type of driver, a shunt switch 22 performs a shorting function in order to implement dimming control.
In the shunt driver design, the shunt switch is controlled by a controller integrated circuit (not shown) in order to provide a duty cycle which provides a desired light output. In a widely used implementation, the shunt switch is controlled per period of the time-varying input signal. This shunt control signal needs to be timed with the frequency of the time-varying input signal e.g. the high frequency signal from an electronic ballast and for this purpose a detection signal is used for timing control. This mains detection signal for example is based on detection of the current flowing from the ballast. A convenient detection signal is the zero crossing of the time-varying input signal which is an AC signal.
A key challenge of new LED drivers is to reduce the size of driver, particularly as more and more functions are added to the driver, such as the Internet of Things and sensor integration. A reduction in the size of the driver is needed in order to provide space for function blocks for these additional functions.
A problem with existing driver designs is the need for a big capacitor to filter the low frequency ripple current for LED lighting.
For some ballasts such as electronic ballasts in North America, the ballast output is not a AC signal with a fixed peak amplitude but with varying peak amplitude. A normal peak amplitude AC signal is for normal burning of the fluorescent lamp; and a periodic higher peak amplitude AC signal is for igniting the fluorescent lamp. The varying frequency is much higher than a control loop for determining the duty cycle of the shunt switch. In other words, the shunt switch has a fixed duty cycle during the varying peak amplitude thus the output after the shunt switching is also varying. This requires a large output capacitor to smooth the varying output. This problem is discussed further below with reference to FIG. 6.